U.S. Pat. No. 2,971,853 (Stookey) discloses photonucleable glasses that are capable of being chemically sculptured and which can then be converted into crystal-containing glass articles upon subsequent heat-treatment. That is, the glasses are susceptive to the development of opacification in selected zones thereof due to the generation of crystals therein via the selective radiation of those zones by shortwave (commonly ultraviolet) radiation followed by heat treatment. These glasses are then treated with a dilute aqueous hydrofluoric acid solution which selectively dissolves the crystallized regions leaving only the glass zones. The remaining glass is then re-exposed to the shortwave radiation and reheated, i.e., cerammed to generate crystals.
Currently, the above described glass article forming process, as commercially utilized, only allows a dimensional control of the centerline specification as precise as .+-.0.0015 inch/inch. However, in today's competitive marketplace, some customers demand much tighter dimensional centerline specifications on their products than the present commercial process is capable of producing on a consistent basis. As a result, the current process typically produces a select rate for those products which is below 25%. The cause of this lack of processing control is that the shrinkage the products experience during the final heat treatment is variable as well as, by nature, difficult to control and predict. Because select rates in the 15-25% range are not cost effective, means to increase productivity for these products, i.e., increase the select rate, are always sought. It is to just such a means which the improvement invention disclosed herein is directed.
Accordingly, the primary objective of the present invention was to devise an improved method of making crystal-containing glass articles within precise dimensions, i.e., increase the process's final product dimensional control, which would, in turn, increase the processing select rates for this process.